1. Field of the Invention
This invention relates generally to a fuel inventory monitoring system, and more particularly, to an ultrasonic-based apparatus and method for the detection and characterization of material in a fuel tank including sludge, water, microorganisms and materials of different viscosities within the tank, the characterization information being presented in an easy-to-interpret picture display for use in determining fuel inventory, leak detection, fuel quality, and the like.
2. Description of Related Art
Current methods for the storage of fuels involve the use of large holding and storage tanks. For purposes of the present description, such tanks are referred to as fuel storage tanks or simply fuel tanks, though other representative applications would include sewage holding and treatment tanks and processing tanks for chemical applications. Such fuel tanks are typically buried underground or are otherwise not easy to access, for maintenance and monitoring applications such as taking an inventory of fuel quantity for determining fuel levels in the tank and for leak detection, and characterizing fuel quality for purposes of facilitating cleaning and removal tasks.
In service-station environments, for example, fuel is delivered to fuel dispensers located at ground level from fuel storage tanks. The fuel storage tanks are often large, cylindrical-shaped containers that may be on the order of 20 meters high and 80 to 100 meters in diameter. Due to regulatory requirements governing service stations, fuel storage tanks are required to be encased in a second or outer casing such that the fuel storage tank contains two walls. These tanks are sometimes referred to as “double-walled tanks.” A double-walled tank is comprised of an inner vessel that holds liquid fuel surrounded by an outer casing. A separate fuel storage tank is provided for each fuel type, such as low octane gasoline, high-octane gasoline, and diesel. A pump is used to deliver the fuel from the fuel storage tanks to the fuel dispensers via a main fuel piping conduit that runs beneath the ground in the service station. The fuels contemplated for storage in the tanks include conventional fuels such as gasoline, diesel, and kerosene, as well as newly-developed fuels containing fuel additives such as ethanol and biodiesel.
A common requirement associated with the use of fuel storage tanks is that of sensing or measuring the level of fluid in the tanks, for example, to warn when a tank is full or should be filled, to control the pumping of fluid into or from a tank so as to avoid overflow or pump damage when a tank is empty, and to otherwise control or measure the level of fluid in a tank. Inherent in the process of storing fuel are also known problems that relate to the accuracy of fuel-inventory measurements, generally and retaining fuel quality, in particular. For example, it has long been recognized that the presence of significant amounts of contaminant substances can affect the accuracy of determinations of volume of a fluid from liquid-level measurements. In particular, fuel storage tanks are susceptible to accumulation of water from the delivered product, condensation, damaged fill boxes, bad gaskets, loose fittings and various other non-water/vapor-tight openings.
Contamination of petroleum-based fuels with water has been a commonly encountered difficulty since fuel tanks must be vented to allow replacement of volumes of fuel withdrawn from a tank with the ambient atmosphere in order to avoid developing a partial vacuum in the tank. The ambient atmosphere may be relatively humid, particularly on water-borne vehicles and the temperature differential between the ambient atmosphere and fuel or the fuel tanks (which will often approximate the water temperature) will cause the moisture in the ambient atmosphere to condense to liquid phase. Therefore, substantial quantities of liquid water may accumulate in fuel tanks over a relatively short period of time. Since water has a greater specific gravity than most petroleum fuels, such as diesel fuel, water that enters a fuel tank will generally collect at the bottom of a fuel tank. This leads to the problems of raising the level of the surface of the fuel in the tank, and it causes the water to be trapped at the bottom of the tank since the water generally is non-soluble with the fuel.
Another known recurring problem associated with the sensing or measuring the level of fluid in a tank or other form of container, is that many of the fluids stored in the tank contain or are comprised of substances that leave or form deposits on the inner surfaces of the tank. The deposits themselves may be comprised of solids dissolved or suspended in the fluids or components of the fluids themselves. These deposits, if left untreated, can interfere with or prevent the accurate measurement or detection of the fluid levels.
In the storage of fuel, in particular, it is known that aerobic fungus, bacteria and yeast hydrocarbon utilizing microorganisms will begin to grow at the fuel/water interface. Such sediments will form on the bottom of the tank as the organisms go through life processes. Further, fuel is an organic compound that reacts with air, water, and microbiological growth. It has a relatively short shelf-life and can degrade over time. Thus, when fuel is stored, contaminants often settle out from the fuel. Contaminants that are more dense than the fuel itself generally fall to the bottom of the fuel tank, forming a non-uniform deposit of materials that build up progressively over time and are often referred to collectively as “sludge.” Unfortunately, these processes occur where current fuel supply lines are principally located—at the bottom of the tank. As the layers of sludge and water build towards the fuel supply lines, it can artificially inflate the float-level readings which, in turn can lead to erroneous fuel-inventory measurements. Further, if left untreated, the presence of the contaminants can adversely affect the fuel quality. The fuel may even become un-pumpable and non-combustible, which could have catastrophic consequences to the end user.
Currently there are tank-monitoring systems with application to determining fuel-inventory levels, characterizing the topography and/or volume of the layer of sludge in the fuel tank, and defining the location of fluid-water interfaces. Such systems generally employ different methodologies to accomplish the desired result. These include systems for directly measuring the materials in the tank by inserting mechanical devices into the tank to make representative sample measurements, as described in U.S. Pat. No. 5,408,874 to Fleck et al., and as described in U.S. Pat. No. 5,953,287 to Willacy, et al., for example. There are other vibration-based systems that measure the effect of a known applied force to the tank to determine the volume (and/or topography) of the liquids in the tank, for instance, the elastic wave sensing system described in U.S. Pat. No. 5,456,114 to Liu, et al. Other systems include capturing a representative sample of material from the tank and storing the sample in a holding tank for experimentation and characterization, such as that described in U.S. Pat. No. 6,604,408 B2 to Dosramos, et al. Still other systems known in the art include manually lowering a dipstick into the water/sludge layer as well as infrared-based systems that sense temperature gradients within the tank, between the water/sludge and fuel interface, for example, to determine the depth of the water/sludge layer. These are just representative conventional systems used to generally describe the current technology.
There are known problems and limitations encountered with such current systems, however, that limit their effectiveness in many applications. For example, such systems are prone to provide erroneous results when the fuel contains contaminants such as sludge and water. Further, the conventional systems cannot accurately characterize and display the properties of the various contaminants, such as the presence of microorganisms at the fluid-water interface or the formation of crystals from floating fatty acids, which are likely to develop in the fuel storage tank. As a result, such systems are self-limiting in an environment where multiple contaminants are present and the user must be able to quantify the contaminants for improvement of overall fuel quality.
As a typical example of a conventional tank-monitoring system, by way of comparison to the present invention, consider that shown representatively in FIGS. 1a and 1b. Such a system uses mechanical devices inserted into the fuel tank to determine the amount of fuel in the tank by measuring the relative height of the fuel as compared with the water present in the tank. Reference is made to FIG. 1a, which generally depicts such a system 100 with a representation of a cross-sectional view of a cylindrically-shaped fuel tank 200 containing a level of fuel 300 and in which a representative layer of water/sludge 400 has formed at the bottom of the tank 200. This exemplary conventional system comprises an inventory control probe 500 adapted to span from the top to the bottom of the tank 200. The inventory control probe 500 is further adapted to receive a water float 510 and a fuel float 520, and it further comprises means for restraining each float 510, 520 to be in approximate alignment with the vertical axis of the probe 500, but otherwise freely suspended (i.e., floating) in the liquids present in the tank 200. The vertical position of each float 510, 520 can in turn be used to determine the amounts of water 400 and fuel 300 in the fuel tank 200.
The water float 510 is generally located in proximity to the bottom of the probe 500, where the majority of the water 400 will accumulate, and it comprises a rubber boot (not shown) that will float on the water 400, but not the fuel 300. In this way, the water float 510 is generally in contact with the water/fuel interface 410. Sensors in the probe 500 report the vertical position of the water float 510 through a transmitter 600, thus determining the amount of water 400 in the tank 200 based on how high the rubber boot floats on the probe 500.
The fuel float 520 is generally located in proximity near the top of the probe 500, and it comprises a rubber boot (not shown) that is designed to float only on the fuel 300, such that the fuel float 520 is generally located at the fuel/air interface 310. Again, sensors in the probe 500 report the vertical position of the fuel float 520 through the transmitter 600, thus determining the measured inventory as the amount of fuel 300 in the tank 200 based on how high the rubber boot floats on the probe 500. The ullage (empty space or fuel capacity remaining) 320 is determined by subtracting the measured inventory from the charted capacity of the tank 200.
The information from the sensors on the probe can be displayed graphically for the user. Sensors on the probe 500 relay the relative positions of the water float 510 and the fuel float 520, and that positional-information is transmitted to the display console 700 either wirelessly 800, as depicted in FIGS. 1a and 1b, or via cable connections (not depicted, but well understood). The display console 700 is schematically depicted in FIG. 1b. Typical data output from such a system include the relative amounts of water 400 and fuel 300 in the tank 200, the ullage 320, as well as the positions of the fuel/air interface 310 and the water/fuel interface 410. Also, such a system 100 can be used for leak detection by determining the change of inventory in the fuel tank 200 over specific periods during idle time.
Despite their relative simplicity and ease of use, such conventional systems are known to suffer from certain disadvantages. A significant disadvantage commonly encountered is that the probes and floats conventionally used are susceptible to erroneous readings due to sludge that accumulates on the rubber boots and varnish that accumulates on the probe. In particular, because the probes and floats are designed to report the levels of water and fuel, they generally cannot monitor contamination such as sludge, micro organisms or free floating contaminants, and they cannot detect changes in viscosity or density between materials in the fuel. These readings are then reported to the display console in the form of erroneous float-level readings, which in turn results in erroneous determinations of fuel levels. Another significant limitation of such systems is that the information reported to the display console generally does not provide visual references, for example, the relative quantity of the various contaminants as compared with the fuel and water levels in the fuel tank. Such information is essential when it comes to determining fuel quality and for devising clean-up and remedial efforts to improve fuel quality.
Monitoring and maintaining fuel quality is of paramount importance in any fuel-storage application. This has always been the case for conventional fuel systems, such as gasoline and diesel. Further, fuel quality is perhaps the single most important issue faced by alternative fuel producers, distributors and consumers. The importance of such alternative fuels has become crucial in recent years. However, the current storage and distribution infrastructure for handling mineral-based petroleum products was not designed for the dynamics of alternative fuel constituents as they are introduced, substituted and diluted into the system.
As environmental and economic pressures dictate the formulation of our fuels, and alternative fuel sources, in particular, there will be a negative impact on the fuel handling infrastructure that will ultimately adversely affect fuel quality. Because of the demanding requirements on today's fuel delivery systems, particularly injectors, a clean fuel supply is extremely important.
Oxidation, repolymerization, water, microbiological life, waxing, gelling, stratification and separation all have an impact on the storability and operability of fuel. In particular, fuel storage tanks are susceptible to accumulation of water from the delivered product, condensation, damaged fill boxes, bad gaskets, loose fittings and various other non-water/vapor-tight openings. Water is known to be the major cause of contamination in fuels. Whether it is mineral or biodiesel fuel, water adversely affects its quality. It is further recognized that the addition of alcohol in the form of ethanol into gasoline, as well as the addition of methyl esters in the form of biodiesel into diesel, will contribute to the degradation process. Of course, such fuels are susceptible to the sludge build-up and other organic processes, such as described here and elsewhere, which can adversely affect fuel quality. This even assumes that the fuel has a sufficient quality to begin with. Along with these natural degradation processes, fuel may also become contaminated through the distribution chain where it is handled numerous times before it reaches the consumer.
In addition to the foregoing problems that relate to the storage of all fuels, there are certain unique problems that can be associated with the storage of unconventional fuels, such as those with various fuel-additives that are now becoming more often used. These include the addition of ethanol to gasoline and the addition of biodiesel to diesel fuel.
Ethanol, also known as ethyl alcohol, can be blended into gasoline as an alternative fuel or as an octane-boosting, pollution-reducing additive. As an alcohol, ethanol is miscible with water, which means that water and alcohol will completely dissolve into each other.
The problem with storing ethanol-blended gasoline is that if there is water present in the storage vessel, as there inevitably will be as discussed herein, the water will be absorbed into the blend. This absorption will continue until the ethanol/gasoline mixture is saturated with water. At that point, called phase separation, the water/ethanol molecule becomes heavy and will fall out of solution. The result is a distinct layer of gasoline floating on the water/ethanol layer, referred to as separation. This separation occurs at the bottom of the fuel tank where conventional fuel pick-up lines are located. It is thus feasible that an end user (e.g., an engine) could receive a significant quantity of a water/ethanol mixture that is noncombustible and potentially damaging to the engine.
Biodiesel is a fuel comprised of mono-alkyl esters of long chain fatty acids derived from vegetable oils, animal fats or recycled cooking oil, and thus is manufactured from esterified vegetable oil, animal fat or recycled cooking oil. Transesterification is the process of exchanging the alkoxy group of an ester compound by another alcohol. These reactions are often catalyzed by the addition of an acid or base. In this process, the oil is mixed with alcohol in the presence of a hydroxide catalyst to produce biodiesel and glycerin.
Some of the problems associated with biodiesel include the formation of white flakes or sediments at the bottom of the fuel tank that are mostly monoglycerides or saturated fatty acids produced from an incomplete reaction or the improper washing of the fuel. It is also known that crystals can form in biodiesel as the fuel is cooled. These precipitants will plug filters and ultimately can become unpumpable, which again is potentially damaging to the engine.
The primary benefit of biodiesel is that it contains oxygen, so it burns cleaner than ordinary diesel fuel, which contributes to lower levels of particulate-matter emissions. As a mythel ester, biodiesel has solvency characteristics that can also dissolve accumulated sediments. Thus, it will add to the diminished quality of fuel at the bottom of the tank.
Because of the natural degradation processes associated with the handling and the introduction of biomass material into the fuel supply, there will be a need to monitor fuel quality more closely to predict possible problems and formulate potential remedial actions. Therefore, what is needed is a fuel inventory monitoring system that will be able to identify sludge, water, microorganisms, the formation of crystals, free floating fatty acids and changes in viscosity, as well as to determine ullage and changes in inventory for leak detection and inventory control.
There further exists the need for a fuel-inventory system and method that can accurately account for, and differentiate between various contaminants, such as sludge and water, that may be present in the fuel tank. Further, there exists the need for a fuel inventory system and method that can identify and characterize the various contaminants that have developed in the fuel supply, for purposes of determining the best remedial actions needed to improve the fuel quality. In particular, a system can method that can visually depict the various contaminants in the fuel is desirable. It is to such a system and method that the present invention is primarily directed.